This is an odd remark - apparently AIX/ESA, the AIX version for mainframe hardware was discontinued "in the late 1990s", roughly the same time when IBM started to invest in Linux.

http://www.lookupmainframesoftware.com/soft_detail/dispsoft/...

There was also Amdahl UTS, which was sold to a company UTS Global around 2000 that appears defunct now.

https://en.wikipedia.org/wiki/Amdahl_UTS

So where is this stronghold of AT&T descendant UNIXes on mainframes then?